diff --git a/extension/background.js b/extension/background.js
index c794baa..a882086 100644
--- a/extension/background.js
+++ b/extension/background.js
@@ -273,15 +273,6 @@ async function videoTabPayload(tabId, opened) {
   }
 }
 
-async function openAiSidepanelForTab(tabId) {
-  if (chrome.sidePanel?.open) {
-    await chrome.sidePanel.open({ tabId });
-    return;
-  }
-
-  throw new Error("当前浏览器不支持扩展侧边栏");
-}
-
 const FETCH_TIMEOUT_MS = 15000;
 
 // Obsidian's Local REST API and the cover images written into the vault; a hung request would block the write forever.
@@ -496,7 +487,7 @@ const BACKGROUND_HANDLERS = {
     const tabId = Number(message.tabId || sender?.tab?.id || 0) || 0;
     if (!tabId) throw new Error(MdgErrors.TEXT.noTab);
     // Before any await: sidePanel.open needs the click that sent the message.
-    const openPromise = openAiSidepanelForTab(tabId);
+    const openPromise = MdgErrors.openSidepanel({ tabId });
     // Awaited once settings load; without a handler now, a fast rejection is reported as uncaught.
     openPromise.catch(() => {});
     const settings = await getMergedSettings();
diff --git a/extension/errors.js b/extension/errors.js
index db260ef..378dd9f 100644
--- a/extension/errors.js
+++ b/extension/errors.js
@@ -76,6 +76,8 @@
     netDown: "网络断了",
     // Asking Chrome or the background to open the side panel failed (popup, video page, 摘读记录, 分拣台); 「：原因」 follows.
     sidepanelOpenFailed: "打开侧边栏失败",
+    // The browser has no extension side panel, or turned it off (Chromium's "API Unavailable"): openSidepanel's reason.
+    sidepanelUnsupported: "当前浏览器不支持扩展侧边栏",
     // 继续问 on a 摘读记录 record that is neither a video nor a web page (a chrome:// or extension page).
     notVideo: "这个页面不能读取，不能继续问",
     // The side panel's 停止 ended the reply (the background's port message, the panel's own stop).
@@ -369,6 +371,15 @@
     return send();
   }
 
+  // chrome.sidePanel.open for every entry (background, 摘读记录, 分拣台), called inside the click before any await. A
+  // browser without the API, or with it turned off (Chromium rejects with "API Unavailable"), says sidepanelUnsupported.
+  const openSidepanel = (options) =>
+    chrome.sidePanel?.open
+      ? chrome.sidePanel.open(options).catch((error) => {
+          throw /API Unavailable/i.test(String(error?.message || "")) ? new Error(TEXT.sidepanelUnsupported, { cause: error }) : error;
+        })
+      : Promise.reject(new Error(TEXT.sidepanelUnsupported));
+
   // What chrome.runtime calls throw in a content script whose extension was reloaded (badges.js checks
   // chrome.runtime.id instead).
   const reloaded = (error) => /Extension context invalidated/.test(String(error?.message || ""));
@@ -394,5 +405,5 @@
     else node.removeAttribute("aria-description");
   }
 
-  globalThis.MdgErrors = Object.freeze({ TEXT, LABEL, action, remedies, pageCode, pageText, grantPages, subtitleCode, text, messageOf, fromResp, grantOrigin, noReceiver, pageSilent, tabMessage, reloaded, escapeHtml, setReason, subtitleMissing, aiTimeout, aiStalled, upTagTitle, noAccess, NET_TIMEOUT_MS, netTimeout, CODE, TIMEOUT_STATUS });
+  globalThis.MdgErrors = Object.freeze({ TEXT, LABEL, action, remedies, pageCode, pageText, grantPages, subtitleCode, text, messageOf, fromResp, grantOrigin, noReceiver, pageSilent, tabMessage, openSidepanel, reloaded, escapeHtml, setReason, subtitleMissing, aiTimeout, aiStalled, upTagTitle, noAccess, NET_TIMEOUT_MS, netTimeout, CODE, TIMEOUT_STATUS });
 })();
diff --git a/extension/errors.selftest.js b/extension/errors.selftest.js
index 4a7aeeb..8d75fd2 100644
--- a/extension/errors.selftest.js
+++ b/extension/errors.selftest.js
@@ -83,6 +83,17 @@ assert.strictEqual(E.messageOf(null, "兜底"), "兜底");
   for (const file of [...sources, "note.js"]) {
     assert.ok(!/拿不到字幕|字幕抓取失败[：（]\$\{|无字幕[，。]/.test(read(file)), `${file} words a missing subtitle itself; use MdgErrors.subtitleMissing`);
   }
+  // openSidepanel: Chromium's "API Unavailable" (side panel turned off) and a missing API say the same sentence; any other
+  // rejection passes on as it is.
+  const opens = async (open) => {
+    ctx.chrome.sidePanel = open && { open };
+    return E.openSidepanel({ tabId: 1 }).then(() => "opened", (error) => error.message);
+  };
+  assert.strictEqual(await opens(async () => {}), "opened");
+  assert.strictEqual(await opens(async () => { throw new Error("API Unavailable"); }), E.TEXT.sidepanelUnsupported);
+  assert.strictEqual(await opens(null), E.TEXT.sidepanelUnsupported);
+  assert.strictEqual(await opens(async () => { throw new Error("No active side panel for tabId: 1"); }), "No active side panel for tabId: 1");
+
   console.log("errors selftest ok");
 })().catch((e) => {
   console.error(e);
diff --git a/extension/history/history.js b/extension/history/history.js
index 57c9613..af93706 100644
--- a/extension/history/history.js
+++ b/extension/history/history.js
@@ -476,7 +476,7 @@ const readsFromPage = (g) => isWebPage(g) || Boolean(g.context.videoId && MdgSit
 // click (sidePanel.open needs the gesture), then its tab in this window is focused, or opened, and the panel asked for
 // the conversation as a video's is (askSidepanel).
 function askAboutPage(group) {
-  const opening = chrome.sidePanel.open({ windowId: OWN_TAB?.windowId });
+  const opening = MdgErrors.openSidepanel({ windowId: OWN_TAB?.windowId });
   setStatus(MdgErrors.TEXT.sidepanelOpening);
   void (async () => {
     try {
@@ -782,7 +782,7 @@ els.list.addEventListener("click", (event) => {
     MdgLimits.askSidepanel({ tabId: OWN_TAB?.id, contextRef: group.context });
     const opening = MdgErrors.TEXT.sidepanelOpening;
     setStatus(opening);
-    chrome.sidePanel.open({ tabId: OWN_TAB?.id }).then(
+    MdgErrors.openSidepanel({ tabId: OWN_TAB?.id }).then(
       () => els.status.textContent === opening && setStatus(""),
       (error) => setStatus(`${MdgErrors.TEXT.sidepanelOpenFailed}：${error.message}`, true)
     );
diff --git a/extension/triage/fav-base.js b/extension/triage/fav-base.js
index 8c64bd8..09835fd 100644
--- a/extension/triage/fav-base.js
+++ b/extension/triage/fav-base.js
@@ -375,7 +375,7 @@ let ownTabId;
 function askAi(it) {
   const contextRef = { site: "bilibili", videoId: it.bvid, title: it.title, author: it.upper, url: videoUrl(it.bvid) };
   MdgLimits.askSidepanel({ tabId: ownTabId, contextRef });
-  chrome.sidePanel.open({ tabId: ownTabId }).catch((err) => toast(`${MdgErrors.TEXT.sidepanelOpenFailed}：${err.message}`, true));
+  MdgErrors.openSidepanel({ tabId: ownTabId }).catch((err) => toast(`${MdgErrors.TEXT.sidepanelOpenFailed}：${err.message}`, true));
 }
 
 // Busy button: disabled + aria-busy (tokens.css draws the spinner) + 「…中」 text; false restores it.